1. Field of Invention
The present invention relates to a synchronous rectifying switching power supply for turning on or off a rectifying switch element and/or a free-wheeling switch element on the secondary side of a transformer, synchronously with a switching element.
2. Description of the Related Art
In a conventional DC/DC converter in which a DC input voltage is intermittently applied to a primary winding of a transformer through a high-frequency switching action of a main switching element, so that an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying element to thereby obtain a DC output voltage, a circuit topology such that a MOSFET is used as a rectifying element or a free-wheeling element provided on the secondary side of the transformer so that the MOSFETs are turned on or off synchronously with the switching action of the main switching element, has been well recognized by those skilled in the art as an effective means for reducing power loss in the circuit elements. However, if a parallel running is performed by connecting several (two, for example) synchronous rectifying DC/DC converters to a common load, the following problems occur.
That is, if the loads in the respective DC/DC converters were well balanced but an output voltage in a second DC/DC converter rises for some reason such as load change, a first DC/DC converter allows its built-in control circuit to detect such rise in the output voltage to thereby control for lowering the output voltage, i.e., for narrowing a pulse conduction width of the main switching element. If such control reaches a limit, the main switching element stops operating, so that the output voltage is applied from the operating second DC/DC converter to the output circuit of the non-operating first DC/DC converter, and thus the gate of the rectifying MOSFET is forward biased, thereby resulting in the turning on of the MOSFET. Then, the electric current is allowed to flow into the secondary winding of the transformer from the second DC/DC converter through the MOSFET, so that a core of the transformer gets saturated and thus the secondary winding gets into a state of substantial short circuit, which allows further strong current to flow in the MOSFET, thereby occasionally damaging the MOSFET. On the other hand, the first DC/DC converter continues receiving the current from the second DC/DC converter, so that the rectifying and free-wheeling MOSFETs start self-oscillation, thus causing, though it depends on cases, failures in the elements due to the heat generated thereby.
FIG. 9 is a circuit diagram showing a specific example of such conventional parallel running switching power supply. In FIG. 9, reference numerals 1A, 1B . . . designate DC/DC converters connected in parallel and 3 a DC power source for supplying a DC input voltage Vi to the respective DC/DC converters 1A, 1B . . . , wherein the respective DC/DC converters have the same circuit topology. In the respective DC/DC converters 1A, 1B . . . , reference numeral 5 designates a transformer of which the primary and the secondary sides are isolated from each other. Reference numeral 8 designates a main switching element such as a MOSFET which is connected in series with the primary winding 6 of the transformer 5. The main switching element 8 turns on or off so that the DC input voltage Vi is intermittently applied to the primary winding 6 of the transformer 5 so as to take out AC voltage from the secondary winding 7 of the transformer 5.
Across the primary winding 6 is connected an active clamp circuit 71 comprising a series circuit of an auxiliary switching element 9 including a MOSFET and a capacitor 10. The main switching element 8 and the auxiliary switching element 9 are turned on or off alternately, defining an off period or dead time, respectively. Thus, the magnetizing inductance of the transformer 5, parasitic capacitance of the respective switch elements 8, 9 (see FIG. 10) are allowed to resonate, thereby achieving Zero Voltage Switching at the time of the turn-on and turn-off of the switching elements 8, 9. In the meantime, reference numeral 72 designates a body diode which is connected in parallel with reverse polarity across the drain and the source of the switching element 8. Likewise, 73 also a body diode which is connected in parallel with reverse polarity across the drain and the source of the auxiliary switching element 9.
A MOSFET 11 serving as a rectifying element is connected in series with the secondary winding 7 of the transformer 5, while a MOSFET 22 serving as a free-wheeling element is connected between the series circuit of the secondary winding 7 and the MOSFET 11. The gate of the MOSFET 11 is connected to a dotted side terminal of the secondary winding 7 where a positive voltage is induced when the main switching element 8 turns on, while the gate of the MOSFET 22 is connected to a non-dotted side terminal of the secondary winding 7 where a positive voltage is induced when the main switching element 8 turns off. A series circuit of a choke coil 13 and a smoothing capacitor 14 is connected across the MOSFET 22. By turning on or off the MOSFETs 11 and 12 synchronously with the main switching element 8, an AC voltage generated in the secondary winding 7 of the transformer 5 is rectified, which is further smoothed by the choke coil 13 and the smoothing capacitor 14, whereby a DC output voltage Vo can be obtained from both terminals of the smoothing capacitor 14. In the meantime, reference numerals 75 and 76 designate body diodes each of which is connected in parallel with reverse polarity across the drain and the source of the MOSFETs 11 and 22.
Reference numeral 17 designates a control circuit for monitoring the DC output voltage Vo and varying a pulse conduction width of a drive signal to be supplied to the gate of the main switching element 8 or the auxiliary switching element 9, corresponding to the change in the DC output voltage Vo, thereby stabilizing the DC output voltage Vo through the feedback by the control circuit 17.
FIG. 10 is a circuit diagram of the DC/DC converter 1A which ceased operating due to the difference in output voltage Vo in the parallel running switching power supply of FIG. 9. Here, parasitic capacitances 82 to 85 of the respective switching elements 8, 9 and the MOSFETs 11 and 12 are taken into consideration. Each switching element 8, 9 on the primary side of the transformer 5 is in a completely off-state. The main switching element 8 is connected to a parallel circuit of the body diode 72 and the parasitic capacitance 82, while the auxiliary switching element 9 is connected to a parallel circuit of the body diode 73 and the parasitic capacitance 83, respectively. Further, the second DC/DC converter 1B for supplying the output voltage Vo, which equivalently serves as a voltage source 87, is connected to the secondary side of the transformer 5.
In a state illustrated in FIG. 10, the MOSFETs 11 and 12 start self-oscillation, through Stages 1 to 4 shown in waveform diagrams of FIG. 11. In the waveform diagrams of FIG. 11, the uppermost waveform indicates a drain-source voltage VSR1 of the MOSFET 11, and the next waveform immediate therebelow indicates a drain-source voltage VSR2 of the MOSFET 22, then a inductor current iL flowing through the choke coil 13, and an magnetizing current iLm flowing in the secondary winding 7 of the transformer 5, in sequence.
FIG. 12 shows the equivalent circuit for State 1. Reference Numeral 91 designates a combined capacitance on the primary side of the transformer 5. If the capacitance of the respective parasitic capacitances 82, 83 are denoted by CQ1, CQ2, while the turn ratio of the primary winding 6 to the secondary winding 7 is n:1, then the composed capacitance equals n2(CQ1+CQ2). Further, reference numeral 92 designates a magnetizing inductance of the transformer 5. The State 1 begins after the free-wheeling MOSFET 22 turns on and the rectifying MOSFET 11 turns off. The main switching element 8 and the auxiliary switching element 9 are in an off state. The Voltage VSR1 across the MOSFET 11 is of a sinusoidal waveform due to the resonance associated with a magnetizing inductance 92 and parasitic capacitances 82, 83 and 84.
On the other hand, the inductor current iL in the choke coil 13 decreases linearly, as the free-wheeling MOSFET 22 is in an on state. State 1 ends as the voltage VSR1 across MOSFET 11 decreases to Zero, and then Stage 2 starts.
The equivalent circuit for State 2 is shown in FIG. 13.
The state begins after the MOSFET 11 turns on and the MOSFET 22 turns off. The voltage VSR2 across the MOSFET 22 rises in the slope of a sinusoidal waveform due to the resonance associated with the inductance of the choke coil 13 and parasitic capacitances 82, 83 and 85. The state 2 ends as the voltage VSR2 across the MOSFET 22 becomes Vi/N, and then State 3 starts.
The equivalent circuit for State 3 is shown in FIG. 14.
State 3 begins after the body diode 72 of the switching element 8 turns on to clamp the voltage VSR2 at Vi/N. Reference numeral 93 denotes an equivalent voltage source at that moment. In State 3, the magnetizing current iLm and the inductor current iL increase linearly. The state ends when iL+iLm greater than 0, which results in turning off the body diode 73, and then State 4 starts.
FIG. 13 shows the equivalent circuit for State 4. This circuit is the same as in State 2 except for initial conditions. Whilst the MOSFETs 11 and 22 continue the self-oscillation through the foregoing four stages, this self-oscillation may generate voltage stresses in the MOSFETs 11 and 22, which may result in the degradation of the MOSFETs 11 and 22. Moreover, the self-oscillation frequency is different from the switching frequency. This results in some interference between DC/DC converters 1A and 1B.
A circuit topology for preventing the rectifying MOSFET 11 from turning on during the stop of operation is proposed in for example Japanese Un-Examined patent publication No. 11-8974. The conventional circuit topology is shown in FIG. 15, in which the free-wheeling diode 12 is connected between the series circuit of the secondary winding 7 and the MOSFET 11. Further, a series circuit of the choke coil 13 and the smoothing capacitor 14 is connected across the free-wheeling diode 12, and thus the MOSFET 11 turns on or off synchronously with the switching element 8, thereby rectifying the AC voltage Vs generated in the secondary winding 7 of the transformer 5, and then further smoothing the thus rectified output voltage by the choke coil 13 and the smoothing capacitor 14, whereby a DC output voltage Vo is obtained from both terminals of the smoothing capacitor 14.
It is noted that this conventional circuit is featured by the Zener diode 21 connected in series with the gate of the MOSFET 11. This Zener diode 21 is of such a characteristic that it conducts relative to the xe2x80x9conxe2x80x9d voltage Vson generated in the secondary winding 7 while it does not conduct relative to the output voltage Vo. Thus, even though the first DC/DC converter 1A is not operating, the rectifying MOSFET 11 is not turned on by the output voltage Vo from the second DC/DC converter 1B. Accordingly, it is possible to prevent the electric current from flowing from the second DC/DC converter 1B through the MOSFET 11 into the secondary winding 7 of the transformer 5, thereby avoiding the damage of the MOSFET 11 caused by the saturation of a core of the transformer 5
In recent years, however, needs for DC/DC converters which can meet a wide range of input voltage Vi have been increased on the market. According to the conventional circuit shown in FIG. 15, however, the gate-source voltage Vgs of the rectifying MOSFET 11 varies so sharply that it is difficult to meet such wide-ranging tendency of input voltage Vi.
As follows is a more detailed description of the above problem based on a waveform diagram of FIG. 16. If the turn ratio of the primary winding 6 to the secondary winding 7 of the transformer 5 is denoted by N: 1, and the Zener voltage of the Zener diode is denoted by Vz, then the voltage Vs generated in the secondary winding 7 and the gate-source voltage of the MOSFET 11 are each as shown in FIG. 16. In the meantime, symbol xe2x80x9cTonxe2x80x9d in FIG. 16 denotes an on or conducting period of the switching element 8, while xe2x80x9cToffxe2x80x9d an off or non-conducting period thereof.
Specifically, as the switching element 8 is short-circuited between the drain and the source thereof during the on period (Ton) of the switching element 8, the voltage generated in the secondary winding 7 equals the input voltage Vi times the turn ratio of the primary winding 6 (Vson=Vi/N). Further, the gate-source voltage Vgson of the MOSFET 11 at this point equals the voltage Vson minus the Zener voltage Vz (Vgson =Vi/Nxe2x88x92Vz).
Assuming that the input voltage Vi=100 V, turn ratio N=5, and Zener voltage Vz=17 V, then the gate-source voltage Vgson of the MOSFET 11 becomes 3 V. If the respective DC/DC converters 1A, 1B . . . shall correspond to 150 V input voltage Vo, then the gate-source voltage Vgson of the MOSFET 11 becomes 13 V, which in turn means that if the input voltage Vi is increased to 1.5 times an initial value, then the gate-source voltage Vgson of the MOSFET 11 also is increased to as much as 4.3 times an initial value thereof, thus resulting in an extremely large fluctuation. Accordingly, there occurs a problem that if the input voltage Vi is raised, a MOSFET with the existing withstand voltage characteristic cannot be used as it is, thus leading to difficulties in meeting widely ranging input voltage Vi.
To eliminate the above-mentioned problems, it is, therefore, a primary object of the present invention to provide a switching power supply that can meet wide-ranging input voltage.
It is another object of the invention to provide a parallel running switching power supply which can prevent the self-oscillation of a rectifying element or a free-wheeling element during the stop of operation.
It is another object of the invention to provide a switching power supply which can reduce on-resistance of the rectifying element during the operation, while the rectifying element is prevented from turning on even against output voltage applied from the external during the stop of operation. It should be noted that a technical goal of the invention common with the above-mentioned objects is to prevent the inflow of electric current from the external when the power supply is not operated.
To attain the above objects, a switching power supply of the invention proposes to include a switch element which is connected between a control terminal of a rectifying switch element and a first terminal of a secondary winding where a positive voltage is induced when the switching element turns on; and a voltage regulation element for turning on the switch element by the voltage induced on the first terminal of the secondary winding when the switching element turns on so as to supply the voltage to the control terminal of the rectifying switch element.
When the switching element turns on while the DC/DC converter is operating, a positive voltage is induced on the first terminal of the secondary winding of the transformer. A this moment, the terminal voltage of the voltage regulation element is equal to the voltage across the secondary winding, but becomes higher than a regulation voltage of the voltage regulation element, so that the switch element turns on to thereby apply the voltage across the secondary winding to the control terminal of the rectifying switch element. Thus, the rectifying element turns on immediately so that the power loss can be reduced.
On the other hand, when the output voltage is applied from the external to the output circuit of the non-operated DC/DC converter, the terminal voltage across the switch element is equal to the output voltage and does not reach the regulation voltage of the voltage regulation element, and thus the rectifying element is not turned on. Accordingly, it s possible to prevent the electric current from flowing through the rectifying element of the non-operated DC/DC converter into the secondary winding of the transformer.
Moreover, the voltage across the control terminal of the rectifying element during the on period of the switching element is not affected by the regulation voltage of the voltage regulation element, but only depends upon the voltage across the secondary winding of the transformer. For this reason, even though the input voltage is varied in a widely ranging manner, the voltage across the control terminal of the rectifying switch element is only varied at the same rate, so that the rectifying switch element with the existing withstand voltage characteristic can be easily used as it is.
According to another embodiment of the invention, a switching power supply of the invention proposes to include a switch element which is connected between a control terminal of the rectifying switch element and a first terminal of the secondary winding where a positive voltage is induced when the switching element turns on; a first voltage regulation element for level shifting the voltage induced in the first terminal of the secondary winding during an on period of the switching element, and then applying it to the control terminal of the rectifying switch element; and a second voltage regulation element for defining a maximum voltage level to be applied to the control terminal of the rectifying switch element.
In this case also, when the switching element turns on while the DC/DC converter is operating, positive voltage is developed at the first terminal of the secondary winding of the transformer. At this moment, the voltage across the switch element is equal to the voltage across the secondary winding, but becomes higher than the regulation voltage of the first voltage regulation element, so that the switch element turns on to thereby apply the voltage across the secondary winding to the control terminal of the rectifying switch element with the voltage being level-shifted. Thus, the rectifying switch element turns on immediately so that the power loss as a switching element can be reduced.
On the other hand, if the output voltage is applied from the external when the DC/DC converter is non-operated, the voltage across the switch element is equal to the output voltage and does not reach the regulation voltage of the first voltage regulation element, the rectifying switch element does not turn on. Accordingly, it s possible to prevent the electric current from flowing into the secondary winding of the transformer through the rectifying switch element.
Moreover, when the input voltage is raised in a wide range, a maximum voltage across the rectifying element is defined by the second voltage regulation element, so that the rectifying switch element with the existing withstand voltage characteristic can be easily used as it is. Accordingly, it is able to easily meet widely ranging input voltage while preventing the inflow of electric current.